Introduction

This page provides an overview of the Liverpool Telescope's fully
autonomous robotic systems. The operational implications for the
astronomer ordering observations from this robotic telescope are
addressed chiefly in the Phase 1 and Phase 2
proposal application pages. However, it's recommended users read
this page as well if one wants to gain a more well-rounded
understanding of this telescope's systems.

Although the Liverpool Telescope (LT) is a robotic telescope, by
"robotic" we do not mean that the telescope is operated by
remote control. Instead, the telescope runs autonomously; that is, it
runs by itself with no human being at the controls.

The Robotic Control System (RCS) is effectively a robot user
that reacts to events as they occur. In the traditional, manned
observatory, a duty astronomer must continually be weighing
together many factors during the course of an observing run in order
to make an appropriate decision on the course of action at that
time.

For example:

Weather conditions may force suspension of the observing
program to protect the telescope, later, when conditions improve
the program may resume.

Power outages may force controlled shutdown of computers and
other hardware.

Changes in atmospheric conditions may require on-the-fly
changes to the scheduled program.

RCS Design Drivers

In designing the RCS, a number of factors had to be taken into account:

Reliability: the system will be left unattended for long
periods and so must continue to function through power failures
and periods of lost internet connection without human
intervention.

Fault-tolerance: the telescope system should be able to
continue operating sensibly but perhaps with degraded performance
when instrument and other subsystems become unavailable.

Adaptability: - deployment to other telescope systems is
envisaged so the model needs to be readily extendable to work with
different TCS and ICS configurations.

Configurability: - the RCS needs to be readily configurable in
order to take into account varying operational requirements and
hardware configuration over time without the need for major
software re-writes.

Efficiency: - it should make best use of available time by
taking into account environmental conditions and availability of
system resources - e.g. instruments, autoguider etc in its
operational planning. Every opportunity should be taken to
increase efficiency by performing operations in parallel.

RCS Operating Modes

Scheduled observing mode

This is the primary observing mode. The RCS requests groups of
observations from the scheduler operating on the observing database
(ODB). A plan of execution is then devised by decomposing the set of
required operations - this can include slewing onto target, setting
the rotator configuration, selecting and configuring the
instrument(s), making focus adjustments, selecting an autoguider and
finally requesting the exposures. In building the plan, consideration
is given to which tasks can be performed in parallel, the required
sequencing of observations to be made, the current telescope and
instrument setup, engineering restrictions, time restrictions imposed
by other operating modes and the approach of sunrise. Commands are
sent to the telescope and instrument controllers to implement the
plan. On completion of a group of observations the observing database
is updated to reflect the current execution status of the group and
the accounts are updated to reflect resource usage.

Target of Opportunity (TO) mode

This priority mode of operation is controlled by an external
software agent triggered by event notifications (e.g. GCN). The
external agent can override any scheduled science or real-time
operations and take full control of the telescope for a period by
sending task requests using a special command interface. The TO
control agent provides execution status information back to the
software agent.

Background observing mode

When conditions are too poor to allow the normal science programs
to run, this operating mode is used to monitor conditions to check for
improvement - e.g. seeing/photometricity/cloud cover. A series of
photometric standard targets are repeatedly observed and atmospheric
statistics derived.

Scheduler

The scheduler provides the RCS on request with groups of
observations to perform.

The current scheduler is a simple despatch scheduler which is run
on demand. The set of available groups in the observing database is
searched to generate a candidate list of those for which all observing, timing and implicit constraints are satisied. A weighted score is
generated for each such group using the metrics below. The group with
the highest score is then selected for observation.

This type of scheduler has advantages and disadvantages:

a principle advantage is that in a potentially rapidly changing environment,
the selected group is always the one that is best matched to current conditions.

the main disadvantage is that the group so selected is not neccessarily
selected at the time which is actually best for it, as there is no
look-ahead facility to to allow consideration of varying future conditions
or to examine alternative future selection scenarios presented by making possibly
sub-optimal selection at the present time.

Timing Constraints

All groups have timing constraints detailing when the group
may be performed:

Flexible - can run any time from start date/time until end
date/time

Monitor - can run during any of a number of windows of
specified size and at specified interval between the start and
end dates. A monitor group will only be attempted once per
window.

Phased - can run on any ONE of a set of (usually narrow)
windows between start date and end (cutoff) date. The windows
occur around a specified ephemeris phase.

Fixed - are required to run at a single precise moment and
take precedence over all other types of group. Actually there is
a small window allowed since it would be possible to miss a fixed
group if e.g. the weather suddenly improved shortly after the
group was due to have started.

Interval - repeat observations to be run as soon as possible
after minimum interval since previous observation has elapsed.

Note: the timing "windows" mentioned above refer
to when the group starts its observing run; the finish time is still
defined by the full execution time of the group. For example, given a
group with a 2-hour execution time and a window between 2100-0000UT,
the observations could be made between 2100-2300UT or as late as
2359-0159UT, depending on scheduling competition with other groups,
weather conditions, etc.

Observing Constraints

Groups may have a number of explicit observing constraints supplied
by the user:

Seeing - the worst level of atmospheric
seeing under which a group is permitted to be observed. A group can
be observed in better conditions but not worse. Seeing is determined
from real-time reductions of science and photometric standard frames
as they are obtained. Corrections are automatically applied for
zenith distance and filter wavelength. The seeing value you set in
the observing constraint is compared against the scheduler's
prediction for the what image quality will be acheived in the r-band
taking into account the current airmass of your target.

For example, if you request average seeing conditions, the
scheduler might attempt the observation at any airmass if the
current seeing is very good, but if the seeing is only average it
might reject your target when low on the horizon because it
predicts the conditions will be poor there.

Scheduling is always performed on an assumption of r-band so
you might expect for example, u-band images to show FWHM larger
than the criterion set in the observing constraint. The
seeing constraint is set as a numerical value,
full-width-half-maximum in arcseconds. The scheduler is specified
to attempt an observation when we estimate there is an 80%
probability of achieving the specified FWHM when the group is
started.

Sky brightness - A relative sky brightness
(described in terms of magnitudes brighter than the best case true
dark sky) is predicted from a model based on solar position, lunar
elevation, lunar phase and lunar separation from telescope
pointing. A group is scheduled only if the predicted sky
brightness is darker than the specified criterion for the entire
predicted execution duration of the group.

Hour-angle - it
is possible to specify HA limits for observing a group - ALL
targets in the group MUST fall within the specified limits in
order to be observed. Ignored by FIXED groups.

Airmass -
A maximum airmass can be specified - the target(s) must remain
above the corresponding elevation for the duration of the
group. Ignored by FIXED groups.

Extinction - Photometric
conditions may be specified - these are determined manually, early
in the night and remain in force for the full night. I.e., they
are only a rough guide to the conditions expected for the coming
night.

Implicit Constraints

A number of implicit constraints are imposed by the scheduler:

All targets in a group must be visible above the dome horizon
(typically 20 - 25 degrees depending on any engineering settings)
for the full duration of the group.

Targets must not cross the zenith exclusion zone during an
observation. This is a narrow zone around the zenith caused by speed
limitation of the cassegrain rotator. Minumimum zenith distance is 2
degrees.

A group will not be attempted if the specified rotator setting
will allow the cassegrain rotator to reach a limit (currently the
range of mount angles are: -87 to +87).

The RCS imposes time limits by which groups must have completed
such as for performing calibration observations. Groups will not be
selected if they are expected to overrun into such periods.

Groups will not be selected if they are expected to overrun
into daytime.

A group cannot be selected if the containing proposal's total
time allocation has been used up.

If a proposal is outside of its activation period no groups can
be selected from it.

No target in a group may cross outside any temporary axis range
- these may be imposed for engineering reasons from time to time and
will be advised via the website.

If any observation in a group specifies an unavailable
instrument, or a configuration which is not currently available for
an instrument, or the instrument is impaired,the group cannot be
selected.

If a fixed group is due before a candidate group can be
expected to complete (with a short buffer time to allow slewing onto
target) then that group cannot be selected.

Autoguider - it is possible to specify whether a group MUST use
the autoguider, may use it if available or must NOT use the
autoguider. If the autoguider is reported as unavailable, groups for
which its use is mandatory will not be selected.

Groups which fail to execute for some reason will become
schedulable after a veto period which depends on the reason for
failure. E.g. groups with mandatory autoguider use which have failed
due to autoguider target acquisition problems are vetoed for 1 hour to
avoid repeated failures using up large chunks of time. Groups which
fail through external reasons e.g. bad weather or target-of
opportunity interrupt, are available for rescheduling immediately the
external condition has cleared.

Scheduler Scoring Criterion

These are the factors used to provide scoring metrics for each group in the candidate list:

Proposal science priority.

Repeating groups have a higher priority to one-off groups.

Urgent groups have higher priority.

Transit height fraction of target(s) - ratio of current elevation to highest
elevation in night.

Closeness of matching of actual seeing to that requested.

Times of Operation

Evening

Run-up of the system in the evening is outlined in the following diagram. Note: "ECI" refers to the Engineering Control Interface which is a front-end for the Master Control Program (MCP) that runs the telescope's basic systems. Thirty minutes before sunset the MCP releases its lock on telescope activities, allowing the RCS to issue startup ("Operational on" or "Oper on") commands.

The above sequence includes the following main actions:

Exactly 30 mins before sunset: the Robotic
Control System issues an "operational on" command, which should
lead to the powering up of the system nodes, hydraulics, etc.

At sunset: the RCS initialises the observing system
by opening the enclosure followed by the mirror covers. This process takes up to
5 min. The system will then start obtaining twilight sky flats.

Down to Solar elevation -10°: the telescope
usually spends 25-30 minutes on sky flats. It will then
either take an initial set of standard star observations
(so that sky conditions can be assessed) or it will
immediately start science observations. Science groups with
extremely relaxed sky brightness constraints may be observed
instead of the initial set of standards, or even shortly after the
telescope has opened, i.e. instead of sky flat observations.
However, for most users the sky will be too bright at this time.
For details on sky brightness (and other) observing constraints
please see the Phase2
guidelines and/or the
Sky Brightness web-page.

Morning

The telescope usually obtains sky flats at the end of the
night (to complement those taken at the beginning) during morning
twilight. However, science groups with extremely relaxed sky brightness
constraints may again be observed at this time.